Desulfurization of transition metal alloys

ABSTRACT

There is disclosed a process for desulfurizing transition metal alloys, particularly in a vacuum induction melting process where the absence of slag is desirable. The process is effected by adding any dilute calcium bearing binary alloy such as an alloy of calcium and nickel or an alloy of calcium and aluminum to the transition metal alloy while it is in the molten state and in contact with an atmosphere that is substantially free of oxygen.

United States Patent McCarty June 24, 1975 DESULFURIZATION 0F TRANSITION 3.622.302 11/1971 Hayashi 75/58 3,695,946 10/1972 Demeaux 75/49 METAL ALLOYS 3,767,380 10/1973 Cseponyi 75/58 Michael McCarty, New Hartford, N.Y.

Special Metals Corporation, New Hartford, NY.

Filed: Feb. 27, 1974 Appl. No.: 446,447

Inventor:

Assignee:

US. Cl 75/10 V; 75/49; 75/58 Int. Cl. C22d 7/00; C216 7/10 Field of Search 75/l0 V, 58,49

References Cited UNITED STATES PATENTS 9/l969 Mahin 75/58 4/l97l Miyashita 75/58 Primary Examiner-Peter D. Rosenberg Attorney, Agent, or Firm-Vincent G. Gioia; Robert F.

Dropkin 5 7] ABSTRACT There is disclosed a process for desulfurizing transition metal alloys, particularly in a vacuum induction melting process where the absence of slag is desirable. The process is effected by adding any dilute calcium bearing binary alloy such as an alloy of calcium and nickel or an alloy of calcium and aluminum to the transition metal alloy while it is in the molten state and in contact with an atmosphere that is substantially free of oxygen.

7 Claims, No Drawings 1 DESULFURIZATION OF TRANSITION METAL ALLOYS BACKGROUND OF THE INVENTION It is frequently necessary to desulfurize transition metal alloys to very low sulfur levels. Transition metal alloys are those which contain substantial quantities of the elements iron, nickel and cobalt individually or in combination. Many methods for desulfurizing such alloys are known. Alloys prepared by induction vacuum melting present special problems because slag in an induction vacuum furnace is heated only by conduction which is so inadequate that a solid slag phase frequently forms.

Calcium is known to react strongly with sulfur. However, it reacts more strongly with oxygen. Accordingly, if lime is employed as a desulfurizing agent, a very large amount of lime must be used which becomes a slag phase that is difficult to maintain molten in a vacuum induction furnace.

Current methods for addition of calcium are inaffective because calcium floats on the molten bath due to its low density and therefore it partially reacts with residual furnace atmosphere and volatizes rather than perform its desulfurization function. Regardless of density considerations, elemental calcium cannot be used effectively because it is very volatile (boiling point l,487C at atmospheric pressure), and it is therefore difficult to maintain contact between elemental calcium and molten iron alloys long enough to obtain the desired reaction because the calcium tends to vaporize from the molten metal.

THE INVENTION This invention is a process whereby the desirable properties of calcium as a desulfurizing agent can be used while avoiding its undesirable characteristics. This invention is a process for desulfurizing transition metal alloys by treating a molten transition metal alloy while in contact with a substantially oxygen-free atmosphere with either an alloy of calcium and nickel or an alloy of calcium and aluminum.

The process of this invention provides many advantages over prior art processes. Primarily, the calcium in a dilute calcium-nickel alloy or a calcium-aluminum alloy is not extremely volatile so it can be introduced and maintained in contact with a molten transition metal alloy long enough for it to be effective in reacting with sulfur.

When used in metal in contact with a substantially oxygen-free atmosphere, there is little competition between oxygen and sulfur to react with calcium so that the calcium is used very effectively and sparingly to remove sulfur. The term substantially oxygen-free is used in the context of this description to define atmospheres obtainable in industry such as those in vacuum induction furnaces or in an inert gas blanketed process. Oxygen and sulfur compete for the available calcium in the process of this invention according to the mass action principle. Small, unavoidable residual amounts of oxygen can be tolerated. However, to use the process of this invention most effectively the concentration of oxygen in the atmosphere over the molten metal should be as low as possible.

The process can be further improved by deoxidizing the molten metal prior to treatment with calciumnickel or calcium-aluminum alloys. Depleting the oxygen in the molten transition metal alloy and in the atmosphere above it diminishes the total amount of calcium addition and correspondingly decreases the amount of calcium oxide that must be dealt with after the desired desulfurization is accomplished.

Whether a calcium-nickel alloy or a calciumaluminum alloy is selected will depend upon the character of the metal treated. If small amounts of aluminum will negatively influence the properties of the metal being treated, then a calcium-nickel alloy must be used and vice versa. Since most transition metal alloys can tolerate nickel, the calcium nickel alloy is the preferred desulfurizing medium of this invention. The preferred composition for calcium-nickel additions are those which have an atomic ratio of nickel to calcium of about 5. This atomic ratio is preferred because of the formation of an intermetalic compound that is quite stable so that vaporization of calcium is greatly retarded. This in turn provides high utilization of the calcium for desulfurization of the transition metal alloys.

Although the process of this invention can be effected at the low pressures obtained in a vacuum induction furnace, it is preferred that an inert gas atmosphere be maintained in the furnace. Argon is the preferred inert gas because of its availability and ease of use although other inert gases may be used. Any pressure of inert gas is beneficial in inhibiting calcium volatilization, however pressures of least mm Hg and up to about 760 mm Hg may be used. Pressures of from 300 to 400 mm Hg are preferred.

The process of this invention is especially useful in making alloys containing iron and nickel by vacuum induction melting techniques. The nickel added from the calcium-nickel alloy is of little consequence with regard to the total nickel content of such an alloy, and in any event it can be accommodated for in compositing the alloy to be treated. This invention is especially useful in making alloys called Superalloys, which are alloys predominately nickel.

The process of this invention is effective to reduce the sulfur level of a transition metal alloy quickly from any reasonable starting level to less than 0.005% w.

DETAILED DESCRIPTION OF THE INVENTION Following are a number of examples of processes embodying this invention which are presented to illustrate rather than limit the invention. In Example Idesulfurizing of three similar nickel-iron superalloys was effected in an experimental vacuum induction furnace holding about 15 pounds of material. Each heat was made from virgin materials and in each case an alloy of 10% w calcium in aluminum was employed as the desulfurizing agent. In all cases the alloy was melted in a furnace at a pressure of less than 20 microns, carbon deoxidized and the furnace chamber was backfilled with argon at the indicated pressure before the addition of the calcium-aluminum alloy. A sample of the alloy was taken 5 minutes after the calcium-aluminum alloy was added, and the final product was poured into an ingot mold 18 minutes after the calcium-aluminum alloy was added. Table 1 reports the results of these three heats.

TABLE l-Continued Heat Heat Heat No. I No. 2 No. 3

Calcium added (Z w) 0.015 0.030 0015 Sulfur at meltdown ('1 w) 0.012 0.017 0.014 Sulfur after 5 min. (1 w) 0.010 (1.002 (1.003 Sulfur in lngot (I w) 0.0(ll 0.0005 0.002

The additional heats were prepared in an experimental vacuum induction furnace holding 300 pounds of material. Rather than use virgin raw materials to make a virgin heat for experimentation as was done with the pound heats reported in Table 1, scrap of the same nickel-iron superalloy composition was employed to which about 0.015% sulfur was added. This is the level of sulfur obtained when ordinary, low cost raw materials are used. In each case the desulfurizing medium was an alloy of calcium and nickel containing about 10% w calcium. The desulfurizing treatment was effected by adding enough of the calcium-nickel alloy to provide 0.03% w calcium in the furnace. Argon pressures as indicated were maintained in the furnace. In lngot No. 1 constant pressure of argon was employed; in lngot No. 2 a constant pressure was maintained in the furnace but a small uncontrolled air leak resulted in continuous introduction of some oxygen and nitrogen into the furnace atmosphere; and in ingot No. 3 a constant argon pressure was maintained for a portion of the heat after which a high vacuum was provided within the furnace. Table II reports the results of these experiments.

air leak microns It is evident from the data reported in Table l and Table II that the process of the present invention is effective ,to reduce the level of sulfur in nickeliron superalloys to a significant extent and very quickly. In all of the heats reported, the vacuum induction furnaces operated without the formation of a significant liquid slag phase and totally without the formation of solid slag. Heat No. 2 reported in Table ll illustrates that the process of this invention can be effected in the presence of a small amount of oxygen but that oxygen issomewhat detrimental. It may be noted that the sulfur level was initially reduced significantly and then slowly rose. It is postulated that the increased sulfur in the ingot over the amount present for 30 minutes of treatment was probably due to the release of sulfur caused by the reaction of calcium sulfide with oxygen.

it may be noted from data related to Heat No. 3 that the presence of an argon atmosphere and the presence of a significant vacuum appeared to be about equivalent for maintaining a substantially oxygen-free atmosphere in contact with the molten metal. it would appear that a slightly better result is obtained when an inert atmosphere of argon is employed instead of a vacuum by comparing the results obtained in Heat No. l with the results obtained in Heat No. 3.

A review of all of the data in Table l and Table 11 indicates that the process of the present invention reduces sulfur levels of nickel-iron superalloys to very low levels, below 0.005% w, very quickly whether a calciumaluminum alloy or a calcium-nickel alloy is employed. The data also indicate that a substantially oxygen-free atmosphere may be maintained either by maintaining a vacuum over the metal or by maintaining an inert gas blanket over the metal. ln all cases the problems previously associated with desulfurizing vacuum induction melted heats were avoided; specifically the problems of solid slags and the problem of vaporized calcium were avoided.

I claim:

1. A process for desulfurizing a transition metal alloy comprising maintaining said alloy in a molten state, substantially in the absence of an oxygen-containing atmosphere and in contact with a calcium alloy selected from a calcium-nickel alloy and a calciumaluminum alloy.

2. The process of claim 1 wherein the calcium alloy is a calcium-nickel alloy having a nickel-to-calcium atomic ratio of about 5 or greater.

3. The process of claim 1 wherein the transition metal alloy is deoxidized prior to treating it with said calcium alloy.

4. The process of claim 1 wherein said transition metal alloy is an alloy containing at least one of nickel, iron, and cobalt and said calcium alloy is a calciumnickel alloy.

5. The process of claim 1 wherein treatment is effected in a vacuum melting furnace.

6. The process of claim 5 wherein said furnace contains an atmosphere of inert gas at a pressure of from about mm Hg to about 760 mm Hg.

7. The process of claim 6 wherein said furnace contains an atmosphere of inert gas and a pressure of about 400 mm Hg. 

1. A PROCESS FOR DESULFURIZING A TRANSITION METAL ALLOY COMPRISING MAINTAINING SAID ALLOY IN A MOLTEN STATE SUBSTANTIALLY IN THE ABSENCE OF AN OXYGEN-CONTAINING ATMOSPHERE AND IN CONTACT WITH A CALCIUM ALLOY SELECTED FROM A CALCIUM-NICKEL ALLOY AND A CALCIUM-ALUMINUM ALLOY.
 2. The process of claim 1 wherein the calcium alloy is a calcium-nickel alloy having a nickel-to-calcium atomic ratio of about 5 or greater.
 3. The process of claim 1 wherein the transition metal alloy is deoxidized prior to treating it with said calcium alloy.
 4. The process of claim 1 wherein said transition metal alloy is an alloy containing at least one of nickel, iron, and cobalt and said calcium alloy is a calcium-nickel alloy.
 5. The process of claim 1 wherein treatment is effected in a vacuum melting furnace.
 6. The process of claim 5 wherein said furnace contains an atmosphere of inert gas at a pressure of from about 100 mm Hg to about 760 mm Hg.
 7. The process of claim 6 wherein said furnace contains an atmosphere of inert gas and a pressure of about 400 mm Hg. 